An Ordinary-looking Man一个相貌平常的人

I will never forget the teacher who taught us chemistry in the middle school.He was an ordinary-looking man and always wore a pair of thick glasses,but he had a strange way of making his classes lively and interesting.
One afternoon,the teacher took us to the Chemistry lab.There he taught us the first lesson in chemistry.
The teacher told us to watch carefully everything he did and then asked us to do the same.He poured some vinegar,kerosene and castor oil into a cup.After that,he mixed the three thoroughly.He dipped a finger in it.Then he sucked a finger and gave a smile.At last he handed the cup around the class.Each of us dipped a finger into the mixture,sucked it and made a face.The mixture tasted horrible.
At last we gave the cup back to the teacher.Now he shook his head sadly,"I'm sorry to tell you,"he said,"that you didn't watch carefully enough what I did.The finger I put into my mouth was not the one I had dipped into the cup."